Fix broken animation on two consecutive swipes.

The bug is caused by allowing a second swipe to restart animation in
BackAnimationController. This CL adds a flag to reject incoming gestures
when an uninterruptable transition (e.g. commit transition, cancel
transition) is already in progress.

To prevent an unfinished transition from failing all future swipes, this CL also introduces a timeout mechanism to start accepting gesture again after a fixed period.

Bug: 221394367
Test: Do two quick consecutive swipes and observe the back to home
animation.

Merged-In: Ib969afb72cc15bd268613546f3495008b6bc9125
Change-Id: Ib969afb72cc15bd268613546f3495008b6bc9125